This invention relates to the field of SZ twisting of stranding elements of electric or optical cables and lines in general and more particular to the mechanical design of appropriate twisting tools in view of minimizing the rotating masses as well as the physical extent of the reversal points of the twist direction in the SZ twisted material.
For the SZ twisting of electric cables and lines, apparatus which consists of a first and a second torsion stop, twisting point or twisting closer and at least one twisting head which is arranged between the torsion stops can be used. The stranding elements to be twisted are fed from stationary supply devices being drawn through the apparatus by a pull-off and take-up device. SZ twisting has the advantage that it can be combined with other operations, for instance, with the extrusion of the conductor insulation, with the twisting of several SZ twisted stranding units, or with the extrusion of an outer jacket.
The tools used for SZ twisting heretofore consist as a rule of twisting heads which rotate about the twisting axis and grip the material to be twisted from the outside tensionally in the circumferentially direction. Either the rotary motion of the twisting heads is changed at intervals (DE-OS No. 15 15 730, DE-AS No. 17 65 452, DE-AS No. 22 30 972), or the distance between the twisting heads and/or the torsion stops or twisting closers is alternatingly increased and decreased continuously. (DE-AS No. 21 38 239, DE-AS No. 22 32 329).
In a known apparatus different therefrom for the SZ twisting of stranding elements of electric cables, a pair of rollers, which is stationary relative to the twisting axis, is used as the twisting tool or part of a twisting tool. The driven rollers are arranged on both sides of the material to be twisted, which is conducted in a straight line, and exert a longitudinal as well as a rotary motion on the material to be twisted because their axes are arranged inclined to the axis of the material to be twisted. For the purpose of SZ twisting several conductors, the inclination of the roller axes can be changed, i.e., the angle enclosed between the crossed roller axes changes its sign (DE-OS No. 25 08 385). In these known machines, a difficulty arises in that, during the initial SZ torsioning or the SZ twisting, reversals of the twist or torsion direction occur, which can easily untwist under the influence of the tension. Otherwise, when twisting several stranding elements the danger of the individual stranding elements coming to lie simply parallel between the pairs of rollers at the reversal points of the twist direction exists and in which case the elements are only shifted back and forth in this manner without resulting in the intended twisting effect.
Twisting machines are also known, in which the stranding elements are twisted in a double lay by means of a twisting yoke, and in which the material to be twisted is deflected within the twisting yoke by an angle of approximately 180.degree. by means of a device which consists of several rollers or roller pairs which are arranged in tandem in the line of the twisting yoke and are set at an angle to the travel direction of the material to be twisted (DE-AS No. 15 10 091). Similar deflecting devices can be used, in twisting machines in which the stranding elements are rotated backwards while being led to the twisting closer, to ensure that the back rotation can propagate into the twisting closer even through a deflecting device provided for the stranding elements (DE-AS No. 15 10 136=U.S. Pat. No. 3,196,601; DE-OS No. 2734398).